Additional RAM will provide little benefit in gaming.
You'll get the biggest bang for the buck by upgrading your 9600GT. Your GPU is what is holding your FPS rate down. Right now is a great time to upgrade video cards as Nvidia and ATI are in a price war with the release of ATI's 6870 & 6850. You can pick up a Nvidia GTX 460 1GB under $200 now, which is great for the value. You can get a ATI 6850 for even less.
All of these cards will CRUSH the performance of your 9600GT.